Braking method for a vehicle

ABSTRACT

A braking method for a vehicle is proposed as a safety measure and replacement function if the brake system fails, in particular the X-by-wire brake system of a motor vehicle, in which the vehicle is braked with the help of the transmission by a defined engagement of frictional shift elements until the vehicle comes to rest, such that the combination of frictional shift elements actuated when the vehicle&#39;s working brake system fails does not correspond to the shift logic of a gear during normal driving operation of the vehicle.

This application is a national stage completion of PCT/EP2004/005235filed May 15, 2004 which claims priority from German Application SerialNo. 103 26 839.1 filed Jun. 14, 2003.

FIELD OF THE INVENTION

The present invention concerns a braking method for a vehicle.

BACKGROUND OF THE INVENTION

The ever-increasing complexity of automatic transmissions, automatedmanual-shift transmissions, continuously variable transmissions, dualclutch transmissions or other safety-relevant aggregates makesever-greater demands on the corresponding diagnosis and monitoringsystems. In this context, the diagnosis and monitoring systems areprimarily intended to ensure passenger protection and driving safety;further aims are to ensure that the vehicle remains available for useand to protect the transmission system and its components.

Precisely in so-termed X-by-wire braking systems, with which the drivercan at times no longer intervene directly in the system, the need forsafety is still greater. Particularly in X-by-wire braking systems, theautomobile industry has to comply with certain standards such as DIN19250 or DIN V 0801, since such systems do not correspond to the stateof the art. This entails high expenditure on testing and monitoringsoftware with its corresponding safety and replacement functions.

SUMMARY OF THE INVENTION

The purpose of the present invention is to indicate a braking method fora vehicle, as a safety measure and replacement function in the eventthat the vehicle's working brake system should fail, in particular theX-by-wire brake system of a motor vehicle, the method ensuring safebraking in certain emergency situations.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, a braking method for a vehicle is proposed in which thevehicle is braked with the help of the transmission system by way ofdefined shifts of frictional shift elements (transmission clutches ortransmission brakes) until the vehicle comes to rest, such that thecombination of shift elements used does not correspond to the shiftlogic during normal driving.

According to the invention, when it is recognized that the vehicle'sworking brake system, for example the E-brake (X-by-wire brake) systemhas failed at a time when the brake pedal is actuated, the vehicle isbraked in a controlled manner via the transmission by way of one or moremonitoring functions. To obtain the maximum possible braking gradient ofthe vehicle, starting from the gear engaged in the transmission at thetime, at least one additional frictional transmission element which doesnot correspond to the shift logic of the currently engaged gear isadvantageously actuated. Thus, the resultant braking effect is notproduced by a downshift initiated when the vehicle's working brakesystem fails, but rather by a defined load on the transmission. This cancontinue until the vehicle comes to rest. When the vehicle is nearly oractually at rest, a parking block of the transmission or a vehicleparking brake can also be actuated. It will be clear to those withknowledge of the field that for this, a corresponding actuation systemmust be provided in the transmission or in the vehicle, for example inthe transmission an E-shift (shift-by-wire) with an active electricconnection between the operating element (selector lever) of thetransmission inside the vehicle and the parking block of thetransmission.

Preferably, the frictional shift element additionally engaged comparedwith the normal shift logic is a brake (disk brake, belt brake, etc.)arranged within the transmission or acting on the drive output of thetransmission.

Thus, to brake the vehicle if the vehicle's working brake system fails,the force flow in the transmission incorporates a defined redundancy inthe sense that, by way of a software function, the torque or shiftingpressure at the frictional shift element engaged additionally comparedwith the normal shift logic is regulated preferably in such manner thatthe vehicle's drive wheels undergo maximum or a specified braking.During this, however, the vehicle's drive wheels must not be lockedwhile the current speed of the vehicle has not yet decreased to adefined value close to zero. The shifting pressure or torque to be setcan be determined as a function of the brake pedal actuation forceand/or the ACC-radar sensor (distance control unit) and/or the actualbraking deceleration of the vehicle.

The additional braking of the vehicle by setting a defined torque/slipat the frictional shift element engaged additionally compared withnormal shift logic necessarily results in increased power loss at theactive frictional element, which in a worst-case situation can lead todamage. For that reason, an optional distinction is made via a furtherfunction, between emergency braking (which may cause damage to thetransmission in an extreme case) and normal braking.

In a particularly advantageous variant of the present invention, afurther second frictional shift element (not previously engaged) isactuated when the limiting thermal load that can be sustained by thefirst frictional shift element engaged to block the transmission isreached, in order to avoid damaging the transmission. A shift can alsotake place to another (higher or lower) gear when the limiting thermalload of the first frictional shift element engaged to block thetransmission is reached in such an emergency situation of the vehicle,so that a different clutch combination can come into use for blockingthe transmission in order to brake the vehicle under control.

In another embodiment of the invention, the vehicle can be additionallyor alternatively braked by actuating two frictional shift elements (inparticular brakes) in the transmission or a separate frictional shiftelement acting on the drive output of the transmission, this engagementof two brakes or of the separate shift element not corresponding to theshift logic or shift element combination for normal driving.

The method according to the invention is not limited to use inchange-under-load transmissions; it can be used with continuouslyvariable transmissions, automated shift transmissions and dual clutchtransmissions.

1-11. (canceled)
 12. A braking method for a vehicle, for use as a safetymeasure and replacement function in the event that a working brakesystem of the vehicle fails, in particular a X-by-wire brake system, inwhich the vehicle is braked with help of a transmission by means of adefined engagement of frictional shift elements until the vehicle is atrest, a combination of frictional shift elements engaged when theworking brake system of the vehicle fails, does not correspond to ashift logic of a gear during normal driving operation of the vehicleand, starting from a gear engaged at a time when the vehicle's workingbrake system fails, at least one additional frictional shift element ofthe transmission is engaged in such manner that drive wheels of thevehicle undergo one of a maximum or a specified braking, but such thatthe vehicle's drive wheels are not locked while an actual speed of thevehicle is greater than a defined value, a shifting pressure or a torqueis set at a first frictional shift element engaged additionally comparedwith the normal shift logic, a shifting pressure or torque to be set,being determined as a function of one or more of a brake pedal actuationforce, an ACC-radar sensor (distance control unit) and an actual speedof the vehicle.
 13. The braking method for a vehicle according to claim12, wherein as soon as the failure of the vehicle's working brake systemhas been recognized, the vehicle is automatically braked by means of thetransmission.
 14. The braking method for a vehicle according to claim 12wherein, if the vehicle's working brake system fails, the vehicle isautomatically braked by the transmission when a vehicle brake pedal isactuated.
 15. The braking method for a vehicle according to claim 12,wherein as a function of the vehicle's actual speed, a respective shiftelement combination is engaged, whereby optimum or maximum vehiclebraking is achieved with least possible stress on the transmission orfrictional shift elements of the transmission.
 16. The braking methodfor a vehicle according to claim 12, wherein a shift takes place to asecond additional frictional shift element of the transmission comparedwith the normal shift logic, when a limiting thermal load of a firstfrictional shift element engaged additionally compared with normal shiftlogic is reached.
 17. The braking method for a vehicle according toclaim 12, wherein a shift to a higher or lower gear with a differentcombination of frictional shift elements takes place, when a limitingthermal load of a first frictional shift element engaged additionallycompared with normal shift logic is reached.
 18. The braking method fora vehicle according to claim 12, wherein the vehicle is brakedadditionally or alternatively by actuating one of two frictional shiftelements in the transmission or a separate frictional shift elementacting on a drive output of the transmission.
 19. The braking method fora vehicle according to claim 12, wherein when the vehicle is nearly oractually at rest, one or more of a parking lock of the transmission anda parking brake of the vehicle is automatically engaged.
 20. A brakingmethod for a vehicle having one of a change-under-load transmission, acontinuously variable transmission, an automated shift transmission or adual clutch transmission,the method for use as a safety measure andreplacement function in the event that a working brake system of thevehicle fails, in particular a X-by-wire brake system, in which thevehicle is braked with help of a transmission by means of a definedengagement of frictional shift elements until the vehicle is at rest, acombination of frictional shift elements engaged when the working brakesystem of the vehicle fails, does not correspond to a shift logic of agear during normal driving operation of the vehicle and, starting from agear engaged at a time when the vehicle's working brake system fails, atleast one additional frictional shift element of the transmission isengaged in such manner that drive wheels of the vehicle undergo one of amaximum or a specified braking, but such that the vehicle's drive wheelsare not locked while an actual speed of the vehicle is greater than adefined value, a shifting pressure or a torque is set at a firstfrictional shift element engaged additionally compared with the normalshift logic, a shifting pressure or torque to be set, being determinedas a function of one or more of a brake pedal actuation force, anACC-radar sensor (distance control unit) and an actual speed of thevehicle.
 21. A braking method for a vehicle, for use as a safety measureand replacement function in an event that a X-by-wire brake system ofthe vehicle fails, the vehicle is braked with help of a transmission bymeans of a defined engagement of frictional shift elements until thevehicle is at rest, a combination of frictional shift elements engagedwhen the working brake system of the vehicle fails, does not correspondto a shift logic of a gear during normal driving operation of thevehicle, the method comprising the steps of; engaging at least oneadditional frictional shift element of the transmission, starting from agear engaged at a time when the vehicle's working brake system fails, insuch manner that drive wheels of the vehicle undergo one of a maximum ora specified braking, but such that the vehicle's drive wheels are notlocked while an actual speed of the vehicle is greater than a definedvalue, setting one of a shifting pressure or a torque at a firstfrictional shift element engaged additionally compared with the normalshift logic, and determining one of a shifting pressure or torque to beset, as a function of one or more of a brake pedal actuation force, anACC-radar sensor (distance control unit) and an actual speed of thevehicle.